There are a host of threads about this. Here's the recap: The 17s have more tire options; the 15s are more period correct and ride a little smoother but finding good rubber can be a PITA.
I have 17" wheels with Kumho Ecsta XS 275/40/17s in the front and 335/35/17s in the back. I did notice a significant difference in gained straight-line traction with the Kumho 335s versus the smaller Yokohamas I used to have, but that is partially due to the tire compound. Of course, just to mix things up a little, I am planning to get some 15s, maybe with some Mickey Thompsons, to change the look occasionally. I have driven some cars with the Avons, and, like divorces, they cost a lot because they are worth it
